The world has changed a lot since the House passed its health care bill last fall.

Back then, House Speaker Nancy Pelosi passed the bill with just two votes to spare. If she took the same vote today, she’d have the bare minimum of votes she would need, after the death of Pennsylvania Rep. John Murtha, three House resignations and the defection of the only Republican to vote yes.

Pelosi’s job is holding the line — or converting some earlier opponents from no to yes if she gets any defections. The good news for Pelosi is that there are at least a half-dozen “no” votes that are open to voting yes, maybe more. But who? And what would it take to flip them?

“I just don’t know where they get the votes in the House,” said Pennsylvania Rep. Jason Altmire, a Democrat who opposed the House bill but now says he’d consider supporting the final package. “It’s a huge challenge because ... the people who voted yes would love a second bite at the apple to vote no this time because they went home and had an unpleasant experience as a result of their ‘yes’ vote. I don’t know if there is anybody who voted no that regrets it.”

After a year of competing proposals and controversy, the fate of the president’s health care proposal rests with a small pocket of Democrats who are caught between the demands of party leaders in Washington and the discontent of angry voters back home. Some voted for earlier bills. Others didn’t.

The playing field has changed, as well. In the House, Tennessee Rep. John Tanner, originally a no, has announced his plans to retire. Indiana Rep. Brad Ellsworth, originally a yes, has announced his intent to pursue a Senate bid. And Democrats across the country face Republican challengers who weren’t on the radar back in November.

The key for President Barack Obama and his congressional allies will be keeping someone like Ellsworth on board, even as he gears up for a Senate bid in a conservative state, while flipping someone like Tanner, who won’t face reelection in the fall.

Ellsworth says the Senate bid won’t weigh on his decision-making process — but an impasse over abortion restrictions could. And Tanner says he’s keeping an open mind at this point, but he’s still not a fan of such sweeping reforms.

“I’d like to get something done,” the veteran Democrat said last week. “But I’m not driving this train, so I’ll have to see if I can get on.”

Of all the unanswered questions, abortion remains the largest, as party leaders scramble to corral the final votes. Michigan Rep. Bart Stupak, who authored a tough anti-abortion provision in the House bill, now says he’ll most likely vote no on the package, the Wall Street Journal reported. Stupak says he has 10 to 12 votes that will be with him on a final package. Backers of reproductive rights question that number.

For more than a year, the Senate has been front and center in the health care fight. But it appears party leaders will pursue a path that requires only a simple 51-vote majority to move a big part of the package through the upper chamber. That means the real drama could take place in the House, where Pelosi’s already narrow margin has only gotten thinner since the first vote.

Some House Democrats have expressed an openness to backing a final package that resembles the Senate legislation because they prefer the bill or believe Senate Democrats worked harder to incorporate Republican ideas.

“The process on the House bill was a bad process,” said Washington Rep. Brian Baird, a Democrat who voted no in Round One. “The Senate procedurally did a better job, ... and they had a better [Congressional Budget Office] score.”

Baird now says he’s “totally undecided,” a change that even he would characterize as an upgrade.

The House passed health care, 220-215, back in November. But that margin has since been whittled to 216-214.

Of the 39 Democrats who voted no, 12 are freshmen who might wilt under the full weight of the White House if Obama leans on them in the run-up to a climactic vote. And three — Tennessee Rep. Bart Gordon, Minnesota Rep. Collin Peterson and Missouri Rep. Ike Skelton — are chairmen, giving the speaker leverage to oust them from those posts if they don’t vote with her on final passage.

Gordon voted for the bill on the Energy and Commerce Committee and has since announced his retirement, leading many to speculate that he will switch his vote on the final round. Skelton, on the other hand, faces two well-funded Republican opponents, giving him a good reason to keep voting no.

And outside groups could step up their pressure on marginal Democrats. MoveOn.org blasted freshman Rep. Larry Kissell of North Carolina after he voted against the initial House bill. Others could find themselves in the cross hairs in the buildup to a vote.

In the past month, Democrats who voted against the first bill are back on the fence, while others who supported it won’t guarantee their support for a final measure. Last month, Rep. Earl Pomeroy told reporters back in North Dakota that he isn’t prepared to back either the House bill or the Senate bill.

But even before Democrats lost a crucial Senate seat in Massachusetts, the politics of health care reform have been working against them. And Republicans are doing everything in their power to make a final vote as uncomfortable as possible. The National Republican Congressional Committee launched ads last week against Pomeroy.

“Speaker Pelosi has made it clear that she is willing to sacrifice a few dozen of her members in order to achieve her objective. The question is which of them will draw the short straw and cast a career-ending vote for her sake,” NRCC spokesman Ken Spain said.

The League of American Voters, an outside group that opposes the president’s reform effort, announced a modest television buy Monday that targets 13 Democrats who voted for the House bill. The ads will run against Indiana Rep. Baron Hill, New York Reps. Dan Maffei and Michael Arcuri and West Virginia Reps. Alan Mollohan and Nick Rahall, along with other frequent targets, like Virginia Rep. Thomas Perriello, Ohio Rep. Zack Space and New Mexico Rep. Harry Teague.

“Let’s put it this way: Speaker Pelosi better have the votes; otherwise, the Democrats are seriously in danger of becoming the minority in the House after the November midterm elections,” Thomas Whalen, a professor of social science at Boston University, said Monday in POLITICO’s The Arena. “Unlike the Olympics, there are no silver medals in politics.”

Democratic leaders are expected to put enormous pressure on their colleagues who have decided to call it quits at the end of the year. Many view Tanner as a critical swing vote, given his symbolic importance to fellow Blue Dogs. But Tanner and others have reservations about pursuing reconciliation because it will only inflame an already partisan debate.

“You’ve got to legislate — to use a football analogy — by first downs instead of 80-yard Hail Marys, and I still prefer that,” Tanner said.

“You’re now getting into the silly political season, where it’s not about philosophy or ideology but just politics — I guess it’s always that way,” said Tanner, who won’t have to face voters in the fall for the first time in 20 years. “The closer you get to November, the more the jockeying begins.”